Lots of info offered some pretty good, others not worth the time.  We are a manufacturer of audio components and have been doing equipment repairs for 25 plus years.  We have heard more equipment than probably everyone posting here combined.  That being said:

Forget Harbeth 40.2 - consider the Revival ATALANTE 5 speakers at around $5K.  Better sounding in every way.

I am also hearing very good things regarding the PMC speakers from a  very good source although I have not heard them yet.  I was not a fan of the old designs.

The other speakers I would go listen to would be the Vandersteen Quattro.  You can get amazing sound from them even against setting them up against the wall.  That can be helpful if your wife is involved in this decision, etc.

Happy Listening.

Huh, what? He needs to buy used with 20k for max value?? What??

Speaker: Tekton Epic 15 ($2400, new, made in the USA)

Subwoofer: Tekton Cinema Sub ($1050 new)

Amp: Schiit Freya+ tube preamp ($1000 new) + Schiit Wotan poweramp ($2000 new, made in the USA)

Dac: Schiit Gungnir 2 ($1600 new)

Conditioner: Audioquest Powerquest 707 ($1300 new)

Sum Total, (new, made in the USA): $9350

Balance: $10650

I might just eat my hat if some 300k Daryl Wilson & D’oggastino rig sounded better. He even had 10k left over to get himself a nice suit, go watch some wind ensembles and dance companies.

$20K budget means you will need to buy used to get max value.

For $20K you can buy brand new and have a really good all around speakers. Consider a Coda CSiB Integrated Amplifier and Legacy Audio Focus Speakers. Contact James Thompson at Destination Hifi and I'm sure he would make you a nice package deal delivered to your door. https://destinationhifi.com/products/coda-csib-integrated-amplifier

I should start with speakers. This I know.

@toontrader I’m not sure this is actually the best approach. But here are some tips.

The first thing you need to sort out is your budget, since if the speaker is inefficient you’ll need a lot of power and a powerful power amp that actually sounds like real music can be an expensive proposition.

So this points to the idea that perhaps a more efficient speaker would be a good move.

The other thing to keep in mind is that the lower the impedance of the speaker, the harder it is to drive, the more critical the speaker cables become and the more distortion the amp (regardless of technology) will make.

So again from a simply ’best sound’ point of view a speaker of higher efficiency and moderate impedance is a good idea. Distortion in the amplifier obscures detail and can make the system sound harsher.

Inefficient speakers have another problem known as ’thermal compression’. This is where the voice coil heats up due to the power being put through it; as it heats up it becomes less efficient and its harder to put current through it. So the speaker tends to compress, which is to say becomes less dynamic.

Your room is on the larger size of things so again efficiency is important.

Since you are already considering horns another advantage is that amps of less power often tend to sound better, especially if the amp is a tube amp.

Here’s something else to consider:

In many rooms you get a phenomena known as a standing wave. This is where the bass waveform is so long that it can bounce off the rear wall and coming forward, can cancel itself. No amount of room correction can fix this sort of thing because room correction depends on amplifier power and if its being canceled, even 1000 Watts won’t fix it. The solution for this is multiple subs asymmetrically placed so to break up the standing wave.

Audiokinesis makes a sub designed for this called the Swarm. Essentially they are placed against the wall in various spots and as long as they are not active above about 80 Hz will not attract the attention of the ear. This can work because at 80 Hz the waveform is 14 feet long, which means by the time your ear has sorted out what note is being played, the bass note has bounced all over the room and is entirely reverberant. So you can use a mono signal to drive all the subs.

The advantage of this is that the main speakers don’t really have the plumb the bottom octave. If they roll off at 40-50Hz you’ll have no problems blending them with the subs. This means there are a lot of smaller speakers that will be easy to place or you can use horn speakers that have horn loaded woofers (which in most cases, don’t go much below 60Hz).

Good Luck with the quest!

Some of the responses here are extremely helpful. In my experience with tube amplification Harbeth speakers didn’t sound nearly as compelling as compared to single driver or horn speakers. I endorse the idea of trying to listen to a lot of speakers before buying and I would suggest you hear DeVore O96 speakers. They are marvelous with tubes! Good luck!